Excellent idea for the stairs. I'll make a few pieces that will rest easily on the stairs.
In the meantime, someone posted over on the Speakeasy, wanting to see how it looked for gameplay with the walls removed. So here are some "exploded" shots of the building.
First up, the front of the museum.

Here is the front of the museum with the centerpiece out. Excuse the tape on the stairs, the glue is still drying.

And here is the back of the museum.

And the back of the museum with the back wall off.

And here is the 2nd floor with the roof pieces off. Those center walls can slide out so as to make one long gallery instead of two smaller ones.

And the first floor with the second floor removed.

Now, I could have built it so that each wall was one piece and instead of taking floors off, I took a wall off. But, no matter how you do that, there will always be some difficulty in getting to the figures. I went with the magnets, not because I wanted to be able to remove walls during play, but because it would make transport and repurposing the walls easier.